1. Field of the Invention
The disclosed technology relates to optical sensors, more in particular to optical tactile sensors integrated in a flexible and/or stretchable foil.
2. Description of the Related Technology
Multiple research centers are developing miniaturized sensor elements that can be distributed over an area to measure physical properties such as pressure, temperature or the proximity of objects. Such miniaturized sensor elements can be fabricated based on microelectromechanical systems (MEMS) technology. Most MEMS devices are built on rigid substrates such as silicon wafers or glass wafers. However, it can be advantageous to provide such sensors on flexible substrates, because this may allow mounting the sensors on non-planar surfaces or even on flexible objects such as a human body. A possible way for providing sensors on flexible substrates comprises flip-chip mounting of a rigid substrate comprising sensors onto a flexible carrier and subsequently selectively etching the rigid substrate for forming distinct rigid islands. A disadvantage of this approach is that it relies on standard IC fabrication technologies, such that it is difficult to create large area sensors at low cost.
Elastomer tactile sensitive foils have been developed based on piezo-resistive, piezoelectric and capacitive force sensing technologies. However, most of these structures suffer from various limitations such as low spatial resolution, small resolution or expensive manufacturing processes.
To circumvent limitations related to electrical connectivity, replacement of electrical sensors by optical sensors clearly offers a solution. Optical technologies have the additional advantage that they are insensitive to electromagnetic interference and that they can be used in harsh environments.
The most common tactile sensors available today are static and passive. They can be used to provide only a static perception of an object shape. Less common are dynamic and active sensors. Such sensors can be used in conjunction with relative motion between a sensor and a contact body to provide a dynamic perception of high frequency elements such as e.g. a surface texture.
In U.S. Pat. No. 7,355,720 a dynamic and active optical displacement sensor is described that can e.g. be used as a vibration sensor or as a pressure sensor. The sensor utilizes optical feedback into a vertical-cavity surface-emitting laser (VCSEL) from an external optical cavity formed by a moveable membrane and an output mirror of the VCSEL. This results in the optical cavity of the VCSEL being coupled to the external optical cavity. As a result, any movement of the membrane in response to sound, vibration, acceleration, pressure, etc. produces a change in the lasing characteristics of the VCSEL, and in particular a change in the intensity of a beam of lasing light produced by the VCSEL (self-mixing interference). This change in intensity is sensed by a photodetector. The fabrication process for the sensors described in U.S. Pat. No. 7,355,720 is relatively complex. The sensors comprise a membrane that is suspended on a rigid substrate. The VCSEL is mounted on another (rigid) substrate that is to be bonded to the substrate comprising the membrane. A good alignment between the photodetector and the VCSEL is needed. The need for suspending the membrane requires a rigid structure (e.g. substrate, spacer) surrounding the sensing element. When forming a sensor array, such rigid structure may limit the sensor density and thus the spatial resolution that can be realized.